Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 42 freshwater gastropod species confirmed for the Atlantic drainages of Delaware, Maryland, New Jersey, Pennsylvania, and the West Virginia panhandle. The management will not be responsible for any loss, damage, or injury resulting from its application elsewhere.

1a) Operculum present. . . Subclass Prosobranchia (2)
1b) Operculum absent . . . Subclass Pulmonata (18)

2a) Operculum calcareous . . . Bithynia tentaculata
2b) Operculum corneous . . . (3)

3a) Operculum multispiral [photo] . . . (4)
3b) Operculum concentric [photo] . . . (5)
3c) Operculum paucispiral [photo] . . . (9)

4a) Shells typically almost planispiral, bearing two carinae . . . Valvata bicarinata
4b) Shells bearing 3 carinae . . . Valvata tricarinata

5a) Shell with color bands . . . Viviparus georgianus
5b) Shell unbanded . . . (6)

6a) Adults strikingly large, shells greater than 40 mm standard length . . . (7)
6b) Adults smaller . . . (8)

7a) Shell uncarinate, aperture lip typically purple . . . Cipangopaludina chinensis
7b) Shell carinate, especially noticeable in juveniles, aperture never
purple . . . Cipangopaludina japonica [photo]

8a) Shell smooth . . . Campeloma decisum
8b) Shell bearing fine spiral cords . . . Lioplax subcarinata

9a) Adults over 10 mm shell length . . . Family Pleuroceridae (10)
9b) Adults under 10 mm shell length . . . (13)

10a) Shell height less than 1.7 times width . . . Leptoxis carinata
10b) Shell height greater than 1.7 times width . . . (11)

11a) Shell bearing many fine spiral cords, at least on
early whorls . . . Pleurocera virginica
11b) Shell with just a single carination,
typically becoming obsolete on later whorls . . . Pleurocera proxima

12a) Inhabits soft water, apex typically decolate . . . Pleurocera proxima (extralimital)
12b) Inhabits hard water, apex complete . . . Pleurocera shenandoa (extralimital)

13a) Primarily terrestrial in life habit, found above the
water level . . . Pomatiopsis lapidaria
13b) Entirely aquatic in life habit . . . Family Hydrobiidae (14)

14a) Penis with a single duct . . . (15)
14b) Penis with two ducts . . . (17)
14c) Males rare or entirely absent, females parthenogenic . . . Potamopyrgus antipodarum

15a) Penis simple, unlobed . . . Somatogyrus pennsylvanicus
15b) Penis fringed with numerous papillae . . . Littorinidops tenuipes
15c) Penis with two accessory lobes . . . (16)
15d) Penis with three accessory lobes . . . Fontigens bottimeri

16a) Proximal penial lobe containing tubular gland . . . Fontigens nickliniana
16b) Proximal penial lobe containing bulbous gland . . . Fontigens orolibas

17a) Operculum entirely paucispiral . . . . Amnicola limosa
17b) Operculum initially multispiral, becoming paucispiral . . . Lyogyrus granum

18a) Shell dextral . . . Family Lymnaeidae (19)
18b) Shell sinistral, not planispiral . . . Family Physidae (22)
18c) Shell sinistral, planispiral . . . Family Planorbidae (26)
18d) Shell patelliform . . . Family Ancylidae (32)

19a) Adult shell length 10 mm or less . . . Lymnaea humilis
19b) Adult shell length greater than 10 mm . . . (20)

20a) Aperture less than 60% shell height . . . Lymnaea elodes
20b) Aperture greater than 60% shell height . . . (21)

21a) Populations on rocks & solid substrates in lakes and rivers . . . Lymnaea catascopium
21b) Populations amphibious, on emergent vegetation . . . Lymnaea columella

22a) Penis lacks a preputial gland . . . Aplexa elongata
22b) Penis bearing a preputial gland . . . (23)

23a) One-part, muscular penial sheath . . . Physa acuta
23b) One-part, glandular penial sheath . . . Physa vernalis
23c) Two-part penial sheath . . . (24)

24a) Shell slender, body generally dark . . . Physa carolinae
24b) Animal lightly pigmented, shell with shoulders more pronounced . . . (25)

25a) Adults greater than 7 mm shell length, whorls convex . . . Physa gyrina
25b) Adults less than 7 mm shell lenght, whorls not notably convex . . . Physa pomilia

26a) Adult greater than 10 mm shell diameter . . . (27)
26b) Adult less than 10 mm shell diameter . . . (29)

27a) Adult lip flared . . . Helisoma campanulatum
27b) Adult lip not flared . . . (28)

28a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps
28b) Spire appears flattened when viewed from one aspect . . . Helisoma trivolvis

29a) Shell covered with periostracal hairs . . . Gyraulus deflectus
29b) Shell bare, with rounded periphery . . . (30)
29c) Shell bare, periphery carinate . . . (31)

30a) Adults smaller than 4 mm maximum shell dimension . . . Gyraulus parvus
30b) Adults larger than 4 mm maximum shell dimension . . .Planorbula armigera

31a) Carina strong, approximately mid-whorl . . . Promenetus exacuous
3b) Carina weak, distinctly off mid-whorl . . . Menetus dilatatus

32a) Apex entirely smooth . . . Laevapex fuscus
32b) Apex bearing fine radial striae . . . (33)

33a) Apex distinctly to the right of the midline [photo] . . . Ferrissia fragilis
33b) Apex approximately in the midline [photo] . . . Ferrissia rivularis